The invention relates to a method, and an apparatus for implementing the method, for shaping and depositing a tire belt ply on a laying drum.
The manufacture of radial carcass tires makes it necessary to deposit, generally during the step of producing and assembling the uncured tire, a set of belts whose function is, amongst other things, to provide the clamping of the crown of the tire. These belt plies are formed from rectilinear cords, often metallic, coated in a rubber mixture, disposed parallel to each other and forming a given angle with the longitudinal direction of the ply.
The requirements of productivity and quality have for many years been leading tire manufacturers to seek a way for automating these laying operations. Thus the practice which consists of first depositing the belt plies on generally cylindrical laying forms and manually adjusting edge to edge the front and rear edges of the ply, and then gripping the crown belt thus produced by means for example of a transfer ferrule and bringing it onto an outer casing previously shaped in the form of a torus, has become widespread.
However, there remains a great difficulty in producing, without the intervention of a skilled operator, an edge to edge joining of the two ply ends with the precision compatible with the quality requirements of modern tires, avoiding superimpositions, offsets or openings.
Several prior art methods for automatically depositing the belt ply on a cylindrical form have been disclosed. These methods describe apparatus and methods whose aim is to control both the ply length, the angular geometry of the ply edge and the positioning of the ply at the time of laying.
Thus, Okuyama et al. U.S. Pat. No. 4,769,104 proposes a method comprising a set of movable arms for aligning and shaping the edge of a ply previously deposited on a magnetic belt. The necessarily rectilinear shape of the arms does not make it possible to correct curved ply edges. In addition, the successive action of the arms tends to modify the previously achieved alignment of the boundary at the time of the alignment of the edge, causing an offset at the bringing together of the two ends of the ply once deposited on the form.
Published EP 0 649 730 reports on a method in which the shape of the ply is analysed by cameras. By comparison with a predetermined theoretical shape, an automatic control system determines the transverse movements to be effected at the time of the deposition of the ply from an unwinding belt to a magnetic belt. This system nevertheless proves ineffective in precisely correcting the angular deviations of the ply edge with respect to a pre-established value.
U.S. Pat. No. 4,857,123 discloses a method in which the geometry of the ply edge is assessed by measuring the difference between the successive positions of a floating blade able to move in the direction perpendicular to the cutting edge, and a predetermined theoretical cutting line. The angular correction is made by a manipulator gripping the ply edge and effecting a controlled rotation about an axis perpendicular to the plane of the ply and passing substantially through the center of the edge, making the toe and heel of the ply undergo identical angular corrections.
U.S. Pat. No. 5,092,946 improves this last method by proposing depositing the ply on a succession of magnetic belts for holding the ply edge in position at each step of the laying cycle. These methods prove relatively effective for correcting rectilinear edges but do not make it possible to juxtapose two ply ends exhibiting a marked curvature of the edges edge-to-edge without causing superimposition, separation or opening.
It will be noted that the difficulties encountered during attempts at automation of the operation of juxtaposition of the edges of two ply ends relate to the very nature of the product, which has a high tendency to vary its length and/or the angle of the cords under the action of a transverse or longitudinal stress. In addition, the release of the stresses after the cutting has a tendency to deform the edge of the ply by curving the cords in a random manner particularly in the area of the toe. The ply is then said to stick-up.